2.1 IMMUNOASSAYS
The identification and quantification of various biological materials using immunoassays offers advantages over common chemical tests, spectroscopic methods, and methods which determine physical parameters (e.g., sedimentation coefficients, electrophoretic mobilities, chromatographic constants, etc.), in that the material can be detected with greater specificity. Immunoassays currently used include radio immunoassays, fluorescent immunoassays, and enzymatic immunoassays. Some disadvantages of the various immunoassay methods currently used include the following: (1) radiolabels used in radio immunoassays have a limited stability and require special handling as well as special means for disposal; (2) fluorescent immunoassays often require separation steps in the procedure, and the fluorescent label may be quenched by a variety of agents present in test solutions; (3) enzyme immunoassays are tedious and can be more difficult to quantitate.
Some recent reports describe the use of liposomes in immunoassays. The liposome membranes are labeled with specific antigens or antibodies and entrap an enzyme or substrate; these liposomes are ruptured in the presence of cognate antibody and serum complement. Release of the liposome-entrapped enzyme can be detected by the addition of the appropriate enzyme substrate to the test solution. However, because the diffusion of macromolecules (such as enzymes) through lesions produced by complement in the liposome bilayer is much slower than the diffusion rate of small molecules the speed and sensitivity of these assays is hampered.